Molecular clouds photoevaporation and FIR line emission

With the aim of improving predictions on far-infrared (FIR) line emission from Giant Molecular Clouds (GMCs), we study the effects of photoevaporation (PE) produced by external farultraviolet (FUV) and ionizing (extreme-ultraviolet) radiation on GMC structure. We consider three different GMCs with mass in the range M GMC = 10 3 -10 6 M . Our model includes (i) an observationally based inhomogeneous GMC density field, and (ii) its time evolution during the PE process. In the fiducial case (M GMC 10 5 M ), the PE time (t pe ) increases from 1 to 30 Myr for gas metallicity Z = 0.05-1 Z , respectively. Next, we compute the time-dependent luminosity of key FIR lines tracing the neutral and ionized gas layers of the GMCs, (C II at 158 m, O III at 88 m) as a function of G 0 , and Z until complete PE at t pe . We find that the specific C II luminosity is almost independent of the GMC model within the survival time of the cloud. Stronger FUV fluxes produce higher C II and O III luminosities, however, lasting for progressively shorter times. At Z = Z , the C II emission is maximized (L C II 10 4 L for the fiducial model) for t < 1 Myr and log G 0 3. Noticeably, and consistently with the recent detection by Inoue et al. of a galaxy at redshift z 7.2, for Z 0.2 Z , the O III line might outshine C II emission by up to 1000 times. We conclude that the O III line is a key diagnostic of low-metallicity interstellar medium, especially in galaxies with very young stellar populations.